Many optical metrology instruments require wavelength-stabilized, single-mode laser sources to operate correctly. Examples are Fourier transform (FT) spectrometers, particle counters and industrial measurement equipment for alignment, positioning, monitoring, or scanning. Many of these instruments have used helium-neon (He—Ne) lasers because of their excellent beam quality and high degree of wavelength stability. Since He—Ne lasers became widely available in the mid-1960s, they have been the only viable choice of laser sources for several decades. This began to change with the advent of the semiconductor diode laser. Starting in the early 1990s, semiconductor diodes began to replace He—Ne lasers in many metrology applications.
By comparison to He—Ne gas lasers, diode lasers are more compact and robust, less expensive, electrically more efficient, radiate less waste heat, and easier to use as they don't require long warm-up times or kilovolts to operate. Overall, laser diodes offer a lower cost alternative for many applications. Until recently, however, they could not be used in products that required extremely high spectral stability and ultra-low wavelength drift due to strong temperature-dependence of the semiconductor material from which they are made. Single-mode diode lasers, such as distributed feedback (DFB) lasers, exhibit a temperature dependence of their optical emission wavelength of about 0.07 nm/° C. (nanometers per degree C.). This temperature dependence alone makes the use of laser diodes difficult and costly in applications requiring a high degree of wavelength stability.
Hence, there is a need in the industry for a method and system for providing temperature compensation to laser diodes in order to achieve a high degree of wavelength stability